The invention relates to a dual-channel electroacoustical transmission system as generally defined hereinafter, which may be classified in the category of "artifical-head" recording or transmissions systems.
The term "artifical head" has generally been used in the literature since 1939 and generally refers to any apparatus having at least two microphones in or on a diffraction body not made of flesh, which is similar in shape or dimensions or volume to a human head. The following publication is incorporated hereinwith by reference. Platte, H.-J.: Zur Bedeutung der Aussenohruber-tragungseigenschaften fur den Nachrichtenempfanger "menschliches Gehor" [On the Significance of Outer-Ear Transmission Characteristics for the Information Receiver Represented by the Human Sense of Hearing]. Dissertation at the Technische Hochschule Aachen, Federal Republic of Germany, 1979.
The basic concept of the artificial head, which has been formulated explicitly only recently, may be summarized as follows: Given the suitable transmission of signals from the human auditory canals or by the eardrums, it would have to be possible to stimulate the eardrums of the test person by means of headsets with the same time functions which occur in the original acoustical field without electroacoustical provisions. Since human hearing "sees" its acoustical environment only by means of these eardrum signals, and with a high-fidelity transmission of the eardrum signals of this kind, then all acoustical perceptions (using the terminology of Blauert, called aural phenomena in the following discussion) must likewise be transmitted in a high-fidelity manner; that is, test persons experiencing a transmission of ear drum signals of this kind would have to describe direction, distances, expansions, spatial characteristics and all other characterics of aural phenomena in exactly the same manner, despite the headset reproduction, as if they were actually located directly in the original acoustical field at the location of the artifical head.
The history of the development of the artifical head over more than four decades indicates that the above-discusssed high-fidelity transmission of aural phenomena has not been attained. In many of the early artifical heads, the causes for this are quite obvious from a present-day viewpoint. For instance, holes having the diameter of 35 mm were simply drilled into the head of a store window mannequin in the vicinity of where the ear was attached. The holes, which at that time received microphones of a corresponding size, naturally caused the acoustical effect of the mannequin auricles to appear different as compared with the effect of an intact natural auricle. Accordingly, the directional hearing ears which have been typical for all subsequent artifical heads already occured with this first artifical head, especially the serious error of front/rear transposition, an effect which was noted by the authors at that time but which could not be explained.
Only in the course of time was it recognized that the acoustical effect of the natural auricle lay in the form of a quite complicated directional characteristic, which is important for the "correct" spatial resolution of the acoustical environment by human hearing. The artifical head was accordingly developed further in the course of time in various laboratories, particularly with a view to better simulation of the natural outer ear and to a reduction in size in the microphones to be used. However, up to 1968 no technology was known in which directional hearing errors of a serious nature did not appear. In 1969 Kuerer, Plenge and Wilkens filed a German patent application No. 1,927,401 entitled "Method for the Aurally Correct Recording and Reproduction of Acoustical Phenomena, and Apparatus for Performing it". The inventors equipped a plastic head with ears which had been simulated comparatively accurately, and a high-quality studio microphone was coupled to each of the ear openings via a special acoustical impedance chip simulating the ear drum. As a result of the work of the company Georg Neumann GmbH, Berlin, in the broadcasting field and especially the broadcast of the first artifical head radio play in 1973, there was great enthusiasm both among the public and among program producers for the new, astonishing, natural transmission technology. Although at first it must have appeared that the problem of artifical-head technology had thus been solved, program producers soon complained about annoying acoustical contamination caused by the artifical head and of what they called a "black hole" in front of the artifical head. What was meant by "black hole" was a virtually conical space whose point is located in the center of the head and the axis of which extends parallel to intersecting lines between the horizontal plane and the plane of symmetry of the head; aural phenomena occur either seldom or never within this conical area when the artifical head signals are reproduced via headsets. This observation, which was usually made by the participants without any awareness of further experiments in the field of artifical heads, showed once again that the earlier typical errors of artifical head technology, specifically the front/rear transpositions, still occurred.
Based on the working hypothesis that all previous artifical heads do not satisfy the purely physically-grounded requirement for error-free transmission of ear signals, prior art techniques used an apparatus of sensor microphones, with which sound signals were transmitted from the auditory canals of a living test person and to those of another living test person in a manner which was satisfactory from the standpoint of measurement techniques. In so doing, the "ideal artifical head" of a living test person was used as a substitute for the artifical heads which had been considered by others to be unsatisfactory. Furthermore all the problems are avoided which occur in coupling microphones to the auditory canal of an artifical head in that the recording of sound was realized by means of introducing a thin sensor into the auditory canal of the test person. In hearing tests it was demonstrated that this apparatus achieves not only the high-fidelity transmission of acoustical signals but also a high-fidelity transmission of aural phemonena and thereby for the first time the probability of front/rear transposition was reduced to an amount which is tolerable and is also unavoidable even in natural hearing, that is, not using electroacoustics.
Although the basic functioning of the artifical head principle had been theoretically demonstrated beyond doubt, hardly a single step had been taken closer to the realization of an artificial head capable of high-fidelity transmission. In the time to follow, further details of the questions of outer ear physics which had still been open or were disputed were cleared up, so that fundamental principles could be derived for a high-fidelity artifical head at least in terms of the transmission of spatial aural phenomena. For instance, it was demonstrated that the acoustical impedance of the eardrum or the acoustical input impedance of the auditory canal on the artifical head which was closed off by the eardrum did not necessarily have to be simulated. Asymmetries were found, which were considered typical, in acoustical transmission behavior between the left and right outer ear in test persons. Such asymmetries have not been taken into consideration structurally in previously known artifical heads. In the prior art it has been demonstrated that the acoustical influence of the upper body on the outer-ear directional characteristic, which had previously been considered negligible can be demonstrated by measurement techniques up to frequencies of 1500 Hz. It has been also demonstrated that the acoustical impedance of the human skin had only a negligibly slight influence on the outer-ear directional characteristic.
In 1980, a new artifical head was developed by altering an earlier artifical head. Even this recent artifical head was demonstrated not to meet the above-discussed requirements for an artifical head capable of high-fidelity transmission in every respect: for instance, the miniature electric microphones, Type BT-1759 of the U.S. manufacturer, Knowles, which were used there preclude a dynamic range which is sufficiently wide for high fidelity and preclude a background noise of the artifical head which is sufficiently low for high fidelity. Since it is known that the coupling of the microphones in the artifical head respresents an essential structural characteristic of an artifical head affecting other structural characteristics and itself being dependent on still further characteristics, the artifical head can in no manner be considered to be an essential, inventive solution of the artifical-head problem, lacking only a little additional expense for the sake of noise reduction. As the later German patent application No. P31 01 264.7 demonstrates, more extensive provisions for improvement of the signal-to-noise ratio in a previously known artifical-head system must be considered equivalent to an inventive new solution to the problem, and the signal-to-noise ratio of the artifical head earlier realized is unacceptable for professional electroacoustical recording purposes.
In hindsight, it appeared that with the known artifical head, the question of the signal-to-noise ratio had finally come to be discussed in artifical-head technology as well. For instance, studies confirmed that a practically usable artifical-head recording system absolutely had to have a greater signal-to-noise ratio than any previously known systems had had. These earlier proposals of using a microphone having an outer diameter of approximately 24 mm for an artifical head and coupling this microphone to the end of a conduit approximately 20 mm in length, however, appear problematic in several respects for the purposes of a practical realization. It can also be inferred from the application of artifical-head sterophonics in television, that the background noise of an artifical head suitable for such a purpose must be as low as possible. The reason is that in order to prevent any discrepancy between the angle of hearing and the angle of seeing, the artifical head must be located behind the camera, and if lenses of a long focal length are used the artifical head must be able to accommodate a long recording distance; that is, the head should furnish electrical output signals which are still uncontaminated by noise despite low acoustical signal levels.
Although more recent work appears virtually to be in the direction of how to construct an artifical head, the invention described herein is the first to succeed in providing a wide-band, low-noise artifical head with a wide dynamic range and having the characteristic of high-fidelity transmission of aural phenomena.